1. Field of the Invention
This invention relates to communication path testing, and, in particular, to methods and apparatus for detecting, from a telephone central office, a subscriber telephone in a receiver-off-hook condition.
2. Description of the Prior Art
A frequent telephone trouble report is where a subscriber reports repeated busy signals when trying to reach a second subscriber. Assuming the first subscriber has dialed correctly, busy signals may be produced by a receiver-off-hook-and-in-use condition, by a receiver-off-hook-and-not-in-use condition, or by a leakage path between the first subscriber's telephone line conductors.
A receiver-off-hook-and-in-use condition may be rapidly detected by voice detecting equipment. When such a condition does not exist, the two remaining possibilities may be distinguished from one another by taking advantage of the nonlinear resistance versus voltage characteristic of a receiver-off-hook telephone and the substantially linear resistance versus voltage characteristic of a conductor-to-conductor leakage path. Apparatus for performing such a function is disclosed in copending application Ser. No. 700,329, filed on even date herewith by R. S. Hoppough.
The Hoppough apparatus applies a controlled-amplitude, alternating test signal at a central office to a subscriber's lines. The controlled amplitude of the test signal prevents elements other than a receiver-off-hook telephone from being driven to the point that they appear as nonlinear impedances. The nonlinear impedance vs. voltage characteristic of the subscriber's telephone in a receiver-off-hook state causes harmonics of the test signal to be generated. A harmonic so generated is detected at the central office, which indicates the presence of a receiver-off-hook telephone.
Although the above-described apparatus provides the desired results under most conditions, there are some conditions under which ambiguities may occur. In some cases it was found that load coils could still present nonlinear appearances. Furthermore, it was found that short circuits in combination with load coils could produce resonant circuit conditions such that the current in a particular load coil could be much greater than the line current supplied by the signal source. Additionally, load impedance at the desired test signal frequency is not always a monotonic function of line length even for modest distances. For example, tests using a 500 Hz test signal were conducted on lines composed of fifteen thousand feet of test trunk with loop lengths varying from three thousand to twenty-four thousand feet. These tests showed that for tip-to-ring shorts, the admittance magnitude as a function of loop length is concave in shape (i.e., not a single valued function). Thus, a third harmonic signal from a short could exceed that from a receiver-off-hook telephone.